Electrodeposition coating compositions and methods are widely used in industry today. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of conductive substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer on all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths usually include an aqueous dispersion of a principal film-forming resin, such as an acrylic or epoxy resin, having ionic stabilization. For automotive or industrial applications for which hard electrocoat films are desired, the electrocoat compositions are formulated to be curable compositions. This is usually accomplished by including in the bath a crosslinking agent that can react with functional groups on the principal resin under appropriate conditions (such as with the application of heat) and thus cure the coating. During electrodeposition, coating material containing an ionically-charged resin is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin, and then applying an electrical potential between the substrate and an electrode of opposite charge. The charged coating material migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure the coating.
Typically, the crosslinker resin does not carry a charge. In order to have the crosslinker resin disperse in the aqueous electrocoat coating bath and migrate to the substrate when the electrical potential is applied, the crosslinker resin is blended with the ionically-charged principal resin to form an organic phase which is then dispersed in the aqueous phase. The charged principal resin then carries the crosslinker that it disperses to the substrate, where the crosslinker is deposited along with the resin. For this to happen, it is important that the crosslinker remain associated with the charged resin. Thus, a crosslinker that is water soluble will dissolve in the aqueous phase will not electrodeposit during the electrocoating process and will not be suitable for preparing curable electrocoat coating films.